1. Field of the Invention
The present invention relates to an optical system and more particularly, to a focus adjustable optical system.
2. Description of the Related Art
In the domain of photography, image-forming lenses of early design commonly have a fixed focus. These image-forming lenses are called fixed focal lenses. However, these fixed focal lenses have a small range of depth of field. During close-up photographing or telephotographing or when the distance between the lens and the object is changed, the object may extend out of the range of depth of field, resulting in a vague image.
In view of the aforesaid problem, auto focal lenses are developed that can automatically adjust the focus to obtain a great range of depth of field. Regular auto focus lens designs achieve auto focus function by moving the whole lens system relative to the CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) image sensor. Because a driving source is necessary to move the whole lens system relative to the CCD (charge-coupled device) or CMOS (complementary metal-oxide semiconductor) image sensor, a big installation space is necessary for accommodating the lens system and the driving source and allowing movement of the lens system. For moving the whole lens system, a high-power driving source shall be used. A high-power driving source for this purpose has a relatively heavy weight. The use of such a high-power and big-sized driving source cannot satisfy the consumers' demand for a small-sized high quality design.